Computer systems operable to convert a multiple-bit image data file to a 1-bit image data file comprising an array of “on” and “off” pixel values for printing on an output medium by an output device are well known. The inventor's white paper “Dot Gain Tool White Paper (v1.1)” discusses such computer systems and output devices in detail and is incorporated herein by reference.
The output medium is divided into an array of notional rectangular, typically square, regions, each region of the medium corresponding to a pixel value of the 1-bit file. In order to print the 1-bit file, the output device attempts to mark each of the notional rectangular regions of the output medium that corresponds to an “on” pixel value of the 1-bit file.
Various factors result in the actual region of the output medium that is marked by the output device when attempting to print an “on” pixel of the 1-bit file differing from the notional rectangular region.
One such factor is that the actual region marked by the output device is typically circular or elliptical, because the output device uses a laser beam (which is circular) to mark the output medium in the case of an output device such as an imagesetter or computer-to-plate system, or squirts droplets of a marking liquid through a slit in the case of an inkjet printer.
Although the diameter of the laser beam of the output device can be adjusted, so as to adjust the diameter of the circular region marked on the output medium when printing an “on” pixel of the 1-bit file, either the circular region will not entirely mark the notional rectangular region, or, if it does entirely mark the notional rectangular region, it will extend into the notional rectangular regions surrounding that region. These situations are shown in FIG. 7 of the drawings.
Where the circular region marked on the output medium does not entirely mark the notional rectangular region, the result is so-called “dot loss” where portions of the image resulting from printing the 1-bit file that are intended to have an image density of, say, 60%, in fact have an image density of, say, 50%.
Where the circular region marked on the output medium is larger than the notional rectangular region, or marks the surrounding notional rectangular regions, the result is so-called “dot gain”, where portions of the image resulting from printing the 1-bit file that are intended to have an image density of, say, 60% in fact have an image of, say, 70%.
Dot loss and dot gain can also result from the way in which output devices that use a laser to mark the output medium operate. Such devices direct a laser device back and forth across the output medium in a so-called “fast scan” direction as the output medium is advanced through the output device or an imaging head is moved across the medium in a so-called “slow scan” direction, turning on the laser device whenever it is necessary to mark one of the notional rectangular regions of the output medium.
The time taken to turn on and turn off the laser device, along with the exposure setting (power level of the laser beam), means that, unless the power output of the laser device is relatively high, the laser device may not be on for a sufficient time to mark the notional rectangular region unless it is one of several consecutive regions to be marked. such that the laser device is already on when it is directed at the region. The turn on and turn off times of the laser device, coupled with the exposure setting, cause dot loss.
It will be appreciated, however, that if the laser device marks several consecutive notional square regions, the resulting region of the output medium that will be marked by the laser device will consist of a rectangular region with a generally semicircular region at either end, resulting from the laser beam being scanned across the consecutive notional square regions. The shape of the regions so marked is shown in FIG. 14. The rectangular region marked on the output medium instead of a series of circular regions causes dot gain.
This has been dealt with by calibrating the computer system by printing a 1-bit file on the output device with a plurality of patches of different image densities from 0% to 100%, measuring the actual densities of the patches printed on the output medium and producing a calibration curve of requested image density against actual image density. The calibration curve is then used by the computer system to adjust the 1-bit file that it generates from the multiple-bit file so that the actual image densities of the image printed on the output medium match the requested image densities.
Although this works well, if the 1-bit file is converted to a multiple-bit file for display by a display device of the computer system, the image displayed on the display device may bear little resemblance to the image that would be printed by the output device. Where the dot gain of the output device is relatively large, as in the case of an inkjet printer that squirts droplets of a marking liquid at the output medium, the image displayed on the display device may be so reduced in image density as to be undiscernible.